1. Field of the Invention
The present invention generally involves apparatus utilized for altering, shaping or grading the surface of ground areas. More particularly, the invention relates to land planes having trippable grader blades and hydraulic systems for steering the plane and raising and lowering the center frame section.
2. Description of the Prior Art
A land plane is a large vehicular device comprised of a wheel supported frame structure which carries one or more grader blades for contacting the ground surface when the plane is towed by a tractor or other suitable tow vehicle. A typical land plane may include a truss-like frame comprised of three distinct sections, a center section and two end sections which are pivotally connected at opposite ends of the center section in such a manner that the end sections may pivot about axes transverse to the longitudinal axis of the plane. This enables the center section, which carries the grader assembly, to be raised and lowered with respect to the ground surface. Pivoting of the end sections with respect to the center section for the purpose of raising and lowering the latter has typically been achieved by using an hydraulic cylinder carried by one end section and operatively connected to the other end section through a cable or rod linkage. While this arrangement is quite suitable for simply raising the center section a considerable distance above ground level in order to accommodate towing the plane to its location of use, it does not easily accommodate precise adjustment of distance between the grader blade edges and ground surface when the plane is set for operation. This is because the frame is usually provided with stops in the form of extensible bolts or similar devices on the center section to adjust for the maximum downward movement of the center section when hydraulic fluid is removed from the lift cylinder. In some instances, it is desirable to dispose the series of grader blades at different distances with respect to the ground surface in order to prevent excess accumulation of soil at the leading blade. Precise adjustment in this regard requires independent control over the pivotal movements of the individual end sections in order to permit corresponding independent adjustment of the stops.
Because of its large wheel base, a land plane requires independent steering of the front and rear wheel assemblies so that steering of the front wheel assembly produces a corresponding though oppositely directed steering action in the rear wheel assembly to accommodate its large turning radius. This has been accomplished by interconnecting the front and rear wheel assemblies through mechanical or hydraulic steering systems. In a typical hydraulic system, an hydraulic cylinder is carried by the front wheel assembly, with the piston rod being actuated by the pivotal movement of a tongue connecting the plane to the tow vehicle. The front cylinder is in fluid communication with a similar cylinder carried by the rear wheel assembly so that steering of the front wheels imparts the corresponding and opposite steering action to the rear wheels. Such a system is also capable of being connected to a separate hydraulic power source, such as that usually associated with the tow vehicle, so that the rear wheels may be selectively steered either through the action of the front cylinder or through the hydraulic power supplied by the tow vehicle. Hydraulic steering systems of this type are normally plagued by entrapment of air in the fluid system. It has been determined that even a small amount of air can cause undesirable variations in steering actions between the front and rear wheel assemblies. Such variations are multiplied manifold in their detrimental effects because of the large turning radius of a land plane.
The grader assembly carried by a typical land plane consists of several blades which are disposed at varying angles to the longitudinal axis of the plane so that the most efficient ground contact can be realized. The blades are usually pivotally mounted so that they can trip rearwardly when obstructions are encountered during grading to prevent damage to the blades. This has been achieved by resiliently biasing the blades with springs or similar appliances so that tripping of the blades will occur when the bias force is exceeded by the force applied to the blades. The bias force is adjustable to compensate for different ground conditions. However, it has been proven difficult to apply a biasing force of such a nature that continuous tripping of the blades during normal operation is avoided while still affording complete protection against damage to the blades.